The present invention relates to a device for detecting infrared rays.
Conventionally, such a device has employed a Schottky silicon type, a silicon impurity doped type, or a compound semiconductor type structure, and the like. Each of these devices has certain disadvantages.
The Schottky silicon type, if used as a photocell, provides a uniform sensitivity and the produciton thereof is easy due to its use of a silicon substrate. However, the device has a narrow sensing range for the incident light and the signal output thereof is low. The silicon impurity doped type has a wide sensing range and the signal output thereof is high, but the production of the substrate therefor is more difficult than for the silicon Schottky type. The compound semiconductor type has a wide sensing range, but the production of the crystal substrate thereof is also more difficult than the Schottky silicon type. Moreover, the leakage current thereof is very large.
A structure such as a silicon Schottky type, in comparison with the other structures, is most suitable for use in a monolithic circuit such as a "CCD" (charged couple device) in a two-dimensional array. However, the greatest disadvantage of this device is a lower sensitivity in detecting infrared rays.
Conventional structures of the Schottky silicon type device, for example, are composed of a metal such as platinum silicide (Pt-Si) or gold (Au) evaporated onto a surface of a P type silicon substrate having a specific resistance of about 10 .OMEGA.-cm. Schottky junctions formed between the metal and the P type silicon substrate can be used for detecting infrared rays.
Usually, the barrier height between Pt-Si and P type silicon is 0.275 eV, and that between Au and P-type silicon is 0.25 ev. The detectable wavelength, based on quantum mechanics, calculated from these barrier values is 4.47 .mu.m in the case of Pt-Si and 4.92 .mu.m in the case of Au. Therefore, the ratio of detecting sensitivity D is usually about 10.sup.9 cm Hz.sup.1/2 W.sup.-1, that is, this value is 1 to 2 orders of magnitude lower compared to that obtainable with other more desirable methods. More specifically, the ratio of detecting sensitivity D (or D*) is defined by ##EQU1## wherein f is an amplifier bandwidth (Hz), Ad is a detector area (cm.sup.2), R is a sensitivity represented in volts per watt (V/W). For the two-dimensional array of the Schottky junction, .DELTA.f=3 Hz, Ad=3.times.10.sup.-5 cm.sup.2, R=1.times.10.sup.9 V/W, and Vn=5 mV, resulting in a ratio of detecting sensitivity D* 1.9.times.10.sup.9 cmHz.sup.1/2 /W.